The clinical characterization of patients with methylmalonic acidemia (MMA) and related disorders, will continue via a dedicated natural history study, NHGRI protocol Clinical and Basic Investigations of Methylmalonic Acidemia and Related Disorders (ClinicalTrials.gov Identifier: NCT00078078), which has enrolled the largest such patient cohort in the world. In the past year, we have focused on the clinical study of MMA related kidney disease, both in the clinic and laboratory. With more aggressive treatment of MMA, long-term complications, particularly chronic kidney disease and renal failure, are becoming more apparent in the surviving patient population. We prospectively assessed renal length, anthropometric measurements, and laboratory evaluations in our large cohort of carefully characterized patients with isolated MMA (Kruszka PS, Manoli I, Sloan JL, Kopp JB, Venditti CP (2013) Renal growth in isolated methylmalonic acidemia. Genet Med, in press). We found that renal length in MMA patients, reflective of kidney growth, was significantly decreased compared to normal controls and derived a multiple regression model to predict kidney growth. In addition to providing the first detailed description of the renal phenotype seen in patients with MMA, this manuscript established a foundation to evaluate renoprotective interventions and their effects on kidney growth and function in the MMA patient population. We have also described an unusual presentation of cblC deficiency as non-immune fetal hydrops (Tanpaiboon P, Sloan JL, Callahan PF, McAreavey D, Hart PS, Lichter-Konecki U, Zand D, Venditti CP (2013) Noncompaction of the ventricular myocardium and hydrops fetalis in cobalamin C disease. JIMD Rep 10:33-8), participated in a review of nutritional management for metabolic disorders, including MMA (Camp KM, et al., (2013) Expanding research to provide an evidence base for nutritional interventions for the management of inborn errors of metabolism. Mol Genet Metab 109:319-28) and participated in the identification of a novel inborn error of metabolism, cblX (Yu H.C., et al., (2013) Mutations in HCFC1 a transcriptional coregulator causes a novel X-linked cobalamin disorder (cblX) with a severe neurological phenotype. Am J Hum Gen. (in press). Active clinical efforts include the characterization of MMA patients who have received solid organ transplantation, neuroradiographic and spectroscopic studies on the MMA stroke syndrome, delineation of the ophthalmological manifestations of MMA and cobalamin disorders, the development of evidence-based dietary guidelines, and definition of the clinical phenotype of CMAMMA. A collaborative intra- and extramural Bench to Bedside grant with Dr Mendel Tuchman (CNMC) to fund our studies using stable isotopes in MMA patients (Metabolic Phenotyping in Methylmalonic Acidemia: Markers and Drug Response (2012-2014) will continue during the next FY. Laboratory investigations have focuses on generating and characterizing mouse models of methylmalonic acidemia and gene therapy studies. Modeling of MMA renal disease in mice has recently been accomplished and correlated with patient findings (Manoli I, Sysol JR, Li L, Houillier P, Garone C, Wang C, Zerfas PM, Cusmano-Ozog K, Young S, Trivedi NS, Cheng J, Sloan JL, Chandler RJ, Abu-Asab M, Tsokos M, Elkahloun AG, Rosen S, Enns GM, Berry GT, Hoffmann V, Dimauro S, Schnermann J, Venditti CP (2013) Targeting proximal tubule mitochondrial dysfunction attenuates the renal disease of methylmalonic acidemia. Proc Natl Acad Sci U S A 110:13552-7). Using the patient experience to guide mouse modeling efforts, a viable model of MMA renal disease was generated by expressing methylmalonyl-CoA mutase (Mut) as a stable transgene in hepatocytes under the control of an albumin (INS-Alb-Mut) promoter. Mut-/-;TgINS-Alb-Mut mice, while completely rescued from neonatal lethality, manifested a decreased glomerular filtration rate (GFR), chronic tubulointerstital nephritis (CTIN) and ultrastructural changes in the proximal tubule mitochondria that were precisely replicated in kidney biopsies from our NIH MMA patients. Moreover, renal physiology studies at the single nephron level were conducted to precisely characterize the nature of the defect and establish that the early and marked reduction of GFR is initiated by proximal tubular mitochondrial dysfunction. Microarray analysis using Mut-/-;TgINS-Alb-Mut kidneys identified numerous biomarkers, including lipocalin-2 (Lcn2), which were validated as associated with renal function in the large NIH MMA patient population. Using the insights from the mouse model and the supporting data from our clinical cohort, we designed a therapeutic study to monitor the response of the GFR to antioxidant therapy in the Mut-/-;TgINS-Alb-Mut mice by inducing the kidney disease with a high protein diet to stress the propionyl-CoA oxidation pathway, with and without vitamin E and ubiquinol in the chow. We demonstrated that the inclusion of antioxidants in the diet could substantially ameliorate the loss of the GFR in the mouse model, establishing the first rational treatment for kidney disease in MMA patients. This study provides the pre-clinical foundation for a future human subjects trial. In the next year, we will continue to create and characterize additional murine models that have an physiologically apparent intermediate or inducible phenotype yet are robust to allow the assessment of gene therapy approaches and the exploration of pathophysiological mechanisms. We will then characterize the disease state using genomic, proteomic and metabolomic approaches to define mechanisms and identify biomarkers that might be translated to patient care. Other than routine dietary and cofactor therapy, no alternative to organ transplantation exists for patients with these inborn errors of metabolism. The successful demonstration of gene therapy for MMA is certain to provide precedence to treat many other disorders of intermediary metabolism, particularly those that feature mitochondrial localization of the metabolic lesion, using a similar approach. Building upon our success with various adeno-associated viral (AAV) vectors to deliver the murine methylmalonyl-CoA mutase gene, we have created new AAVs suitable for potential translation to humans. One affords ubiquitous expression of the human MUT enzyme and another directs hepatic expression. The ubiquitous expression vector has been studied in Mut-/- mice and dose reduction studies have established the minimum dose needed to rescue mice that display a neonatal lethal phenotype. A manuscript detailing these findings has been published (Chandler RJ, Venditti CP (2012) Pre-clinical efficacy and dosing of an AAV8 vector expressing human methylmalonyl-CoA mutase in a murine model of methylmalonic acidemia (MMA). Mol Genet Metab 107:617-9), while experiments to characterize the liver specific expression cassette are ongoing. We will continue to determine the most effective dose and AAV vector for efficient gene therapy in a variety of MMA mouse models, including new transgenic knock-out models. Analysis of the effectiveness of each vector will use growth, metabolic, biochemical, expression and in vivo metabolic studies to determine the degree of correction achieved. It is anticipated that such pre-clinical studies will lead to the creation of an optimal vector for use in humans, affording an opportunity to pursue an IND submission to the FDA as a step toward translating gene therapy to the clinic.